Microwave mixer for mutually orthogonal waveguide modes



Jan. 1, 1963 B. M. SCHIFFMAN 3,071,729

MICROWAVE MIXER FOR MUTUALLY ORTHOGONAL WAVEGUIDE MODES Filed Feb. 16, 1961 INVENTOR. BERNARD M. SCHIFFMAN ATTORNEY 3 ,071,729 Patented Jan. 1, 1963 3,il"7l,72;9 MICROWAVE MHXER FGR MUTUALILY GERTHS ONAL WAVILGIEDE MGDES Bernard M. Schifiman, Mountain View, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Feb. 16, 11961, Ser. No. 89,803 6 Claims. (Cl. 325-446) The present invention relates in general to microwave mixers of the type wherein the microwave frequency waves are supported as mutually orthogonal modes in a single waveguide, and more particularly to novel wave guide structures for improving the energy coupling or impedance match to the diodes of such mixers.

In the copending US. patent application of Richard M. Whitehorn, Serial No. 862,356, filed December 28, 1959, now US. Patent 3,066,290, issued November 27, 1962, there is disclosed and claimed an extremely compact and lightweight microwave mixer consisting of a section of rectangular waveguide supporting two mutually orthogonaldorninant electric modes respectively established by two microwave frequency waves, and having a pair of diodes collinearly disposed therein. Means are provided for perturbing the electric field of one mode to couple to each diode in an opposite direction whereas the electric field of the other mode is unperturbed and couples to each diode in the same direction. Thus, the signals of said diodes may be combined to obtain a balanced beat frequency signal.

In arrangements wherein the unperturbed wave is coupled through a junction connection consisting of a single mode rectangular Waveguide, it has heretofore been the usual practice to make the dimension of the orthogonal mode waveguide transverse to the polarization of the unperturbed wave the same as the width of the connecting waveguide. It has been discovered with respect to such an arrangement that increasing the frequency of the unperturbed wave decreases its coupling to the diodes, thereby placing an undesirable limitation on the operating frequency range. The apparent reason for this loss of coupling is that the dielectric diode cartridges present a heavy capacitive loading to the unperturbed wave so that the cut-off frequency for undesired higher order modes is lower in the orthogonal mode waveguide section than in the connecting waveguide.

It is the principal object of the present invention to provide novel means for overcoming this and other limitations of orthogonal mode mixers.

One feature of the present invention is the provision of an orthogonal mode mixer waveguide section in which the dimension transverse to the polarization of the unperturbed mode is substantially smaller than the width of the connecting waveguide through which said mode is coupled.

Another feature of the present invention is the provision of an orthogonal mixer waveguide section having a rectangular cross-section which is oblong in the axial direction of the diodes collinearly disposed therein.

Another feature of the present invention is the provision of a thin rectangular iris for coupling the unperturbed mode of an orthogonal mode mixer to the connecting waveguide.

Still another feature of the present invention is the provision of a bifurcated iris for coupling the perturbed mode of an orthogonal mode mixer to the connecting waveguide.

These and other features and advantages of the present invention will be more apparent after a perusal of the following specification taken in connection with the accompanying drawings wherein,

FIG. 1 is a pictorial schematic view of a mixer and connecting waveguide configuration in accordance with the present invention,

FIG. 2 is an isometric view of a mixer constructed in accordance with the principles illustrated in FIG. 1,

FIG. 3 is a front view, partially broken away, of the mixer of FIG. 2 looking into the local oscillator waveguide port, and

FIG. 4 is a rear view of the mixer of FIG. 2 looking into the signal waveguide port. 7

Referring to FIG. 1, the mixer waveguide section 1 is energized in two orthogonal modes by a vertically polarized signal wave propagated through a single mode rectangular waveguide 2 connected at a step transition adjacent one end thereof, and a horizontally polarized local oscillator wave propagated through a similar perpendicularly disposed waveguide 3 connected at a step transition adjacent the opposite end thereof. Positioned inside the waveguide section I is a pair of collinearly disposed crystal diode rectifiers 4 and 5 contacted at the oppositelypoled inwardly extending terminals thereof by a horizontal output post 6. The electric field of the signal wave is unperturbed by the post 6 and couples into each of the crystal rectifiers 4 and 5 in the same direction as indicated by the dotted arrows. The local oscillator wave, however, is perturbed by the post 6 in such a manner that the electric field thereof couples into each crystal rectifier in an opposite direction as indicated by the solid arrows. The difference between the currents in the separate rectifiers flows along the conductor 6 through a low-pass insulating choke 6' in the side wall to the inner conductor of a balanced LF. beat frequency coaxial output connector, the current components in the separate crystal rectifiers due to local oscillator noise cancelling each other at the junction of post 6.

Since the dielectric cartridges of crystal diodes 4 and 5 lie in the polarization direction of the signal Wave, they tend to present a heavy capacitive loading thereto which lowers the cut-ofi frequency of higher order modes, thereby degrading the coupling of signal energy to the diodes at the high end of the band of frequencies which may be propagated by the waveguide 2. In accordance with the present invention, this problem is overcome by making the efiective waveguide width a for signal waves in the mixer section 1 substantially (approximately 25-30%) less than the waveguide width a of the connecting waveguide 2. Under these conditions, the cut-off frequency for higher order signal modes in the mixer section 1 is comparable with that in the connecting waveguide 2, thereby enabling the useful frequency range to be eX- tended. For example, making the dimensions at equal to .650" as compared to the standard X-band dimension of .909 enabled the high frequency end of the operating range to be raised from approximately 10.5 kmc. to greater than 12 kmc.

It will be noted that the crystal diodes 4 and 5 are positioned mid-way of the mixer dimension a in order to insure maximum coupling to the signal field, and that the local oscillator connecting waveguide 3 is positioned off-center so as to maximize the perturbing effect of the post 6 for the local oscillator mode. In order to minimize the space occupied by the entire waveguide assembly, the signal waveguide 2 is also placed off-center to line up with the local oscillator Waveguide 3.

In the usual situation where the same size of connecting waveguide is used for both the local oscillator and signal waves, the mixer waveguide section had heretofore been of a substantially square cross-sectional configuration. According to the present invention, however, the crosssection is oblong in the axial direction of the diodes. In addition to the ability to eliminate higher order modes as described above, such oblong configurations are found to be generally advantageous in improving the coupling s aman 3 of both the local oscillator and signal wave to the diodes since the electrical distance from the signal waveguide port to the diodes is relatively shorter for the signal wave, and relatively longer for the local oscillator wave. Thus, the

diodes are placed in the region of strong signal fieldreceiver applications, which operates in the frequency range of to 12 krnc. with standard X-band connecting waveguide. The oblong waveguide section 1 is recessed in a rectangular metallic block 1 provided with a flange plate 10 for mating to a vertically polarizing signal waveguide and a flange plate 11 for mating to a horizontally polarizing local oscillator waveguide. These two connecting waveguides are indicated in phantom as 10 and 11', respectively. The crystal rectifiers 4 and 5 are seated in removable caps 12 threaded through the top and bottom walls, and are held in contacting relation at the inner terminals thereof by a connector 13. The output LF. post 6 is attached to the connector 13, and extends through the insulating low-pass choke 6' in the waveguide wall to form the inner conductor of the output coaxial LF. connector 6". Each cap 12 is provided with a filter network bypassing the AC. crystal current components to the grounded waveguide block 1' so that the DC. current levels in the crystal rectifier may be monitored at the lugs 14 without producing undesirable leakage fields.

A thin, horizontally extending, rectangular iris 21 in the signal waveguide flange Ill, centered with respect to the diode rectifiers, is found to improve the signal VSWR and hence the noise figure. A thin, vertically extending, bifurcated iris 22 in the local oscillator flange 1.1, centered with respect to the local oscillator waveguide 11', also aids in improving the noise figure, especially at higher frequencies, by reducing the protrusion of the signal field into the local oscillator waveguide and hence moving in the effective shorting plane for signal waves, without any substantial degradation in the coupling of local oscillator energy therethrough.

Certain possible modifications of the present invention should be particularly noted. Since the mixers of the present invention are reciprocal hybrid devices, the roles of the various terminals can, under suitable conditions, be reversed. For example, the signal and local oscillator inputs could be reversed; or the mixer operated as a side-band modulator by using the perturbing post as the input terminal for the modulating signal and the opposite waveguide connections for the carrier input and modulated output, respectively. Further, the mixer may be operated as a parametric amplifier by using voltage-controlled diode capacitors instead of dissipative diode rectifiers. Still further, it may be desirable in some instances to establish a waveguide mode of desired polarization by means of a junction connection consisting of a coaxial probe rather than a single mode waveguide.

Since many changes couldbe made in the above construction any many apparently widely diiferent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above construction or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A microwave mixer comprising: a rectangular waveguide section capable of supporting a pair of orthogonally polarized dominant electric waveguide modes established by first and second waves propagating, respectively, through a pair of junction connections adjoining said section, the junction connection for said first wave consisting of a single mode waveguide coupled to one end of said orthogonal mode waveguide section through a step transition at which dimension of said waveguide sec tion transverse to the polarization direction of said first wave is substantially less than the width of said connecting waveguide; 21 pair of collinearly disposed diodes extending inwardly from opposed portions of the wall of said Waveguide section in the polarization direction of said first wave; and means for perturbing the mode established by said second wave so that the electric field thereof extends in an opposite direction along the axis of each diode.

2. A microwave mixer comprising: a rectangular waveguide section of oblong cross-section capable of suppoiting a pair of orthogonally polarized dominant electric waveguide modes established by first and second waves propagating, respectively, through a pair of junction connections adjoining said section; a pair of collinearly disposed diodes extending inwardly from opposed portions of the wall of said waveguide section in the polarization direction of said first wave, the dimensions of said wave guide cross-section in said polarization direction being substantially greater than that in the direction transverse thereto; and means for perturbing the mode established by said second wave so that the electric field thereof extends in an opposite direction along the axis of each diode.

3. A microwave mixer comprising: a section of rectangular waveguide capable of respectively supporting in two orthogonally polarized dominant electric modes, a first wave received through a first single mode connecting Waveguide adjoining one end thereof at a first step transition, and a' second wave received through a perpendicularly disposed second single mode connecting waveguide adjoining the opposite end thereof at a second step transition, the dimension of said waveguide section in the polarization direction of said second wave being substantially smaller than the width of said first connecting waveguide; a pair of collinearly disposed diodes extending inwardly from opposed portions of the wall of said waveguidesection in the polarization direction of said first Wave, the inwardly extending terminals of the separate diodes being of opposite polarity; and a conducting post contacting said inwardly extending terminals and insulatedly extending through an adjacent wall portion of said waveguide section along the perpendicular bisector of the axis of said crystal rectifiers to provide a balanced beat frequency terminal.

4. The mixer of claim 3 wherein said first step transition includes a thin coupling iris extending transverse to the polarization direction of said second wave.

5. The mixer of claim 3 wherein said second step transition includes a thin bifurcated coupling iris extending transverse to the polarization direction of said second wave.

6. The mixer of claim 3 wherein said diodes are centered with respect to said waveguide section dimension and said second connecting waveguide is oil-set in the direction of said conducting post.

References Cited in the file of this patent UNITED STATES PATENTS 2,514,679 Southworth July 11, 1950 2,761,061 Mattern Aug. 28, 1956 2,806,138 Hopper Sept. 20, 1957 2,813,972 Anderson et al Nov. 19, 1957 OTHER REFERENCES Corbell: Microwave Mixer for Airborne Radar, Electronics, February 5, 1960, pages 70 and 71.

UNITED STATES PATENT OFFICE CERTIFICATE OF CQRRECTION Patent No. 3,071 T29 January 1 Y 1963 Bernard M. Schiffman It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4 line 25, for "dimensions" read dimension line 54, for "second" read first Signed and sealed this 20th day of August 1963.,

(SEAL) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents 

1. A MICROWAVE MIXER COMPRISING: A RECTANGULAR WAVEGUIDE SECTION CAPABLE OF SUPPORTING A PAIR OF ORTHOGONALLY POLARIZED DOMINANT ELECTRIC WAVEGUIDE MODES ESTABLISHED BY FIRST AND SECOND WAVES PROPAGATING, RESPECTIVELY, THROUGH A PAIR OF JUNCTION CONNECTIONS ADJOINING SAID SECTION, THE JUNCTION CONNECTION FOR SAID FIRST WAVE CONSISTING OF A SINGLE MODE WAVEGUIDE COUPLED TO ONE END OF SAID ORTHOGONAL MODE WAVEGUIDE SECTION THROUGH A STEP TRANSITION AT WHICH DIMENSION OF SAID WAVEGUIDE SECTION TRANSVERSE TO THE POLARIZATION DIRECTION OF SAID FIRST WAVE IS SUBSTANTIALLY LESS THAN THE WIDTH OF SAID CONNECTING WAVEGUIDE; A PAIR OF COLLINEARLY DISPOSED DIODES EXTENDING INWARDLY FROM OPPOSED PORTIONS OF THE WALL OF SAID WAVEGUIDE SECTION IN THE POLARIZATION DIRECTION OF SAID FIRST WAVE; AND MEANS FOR PERTURBING THE MODE ESTABLISHED BY SAID SECOND WAVE SO THAT THE ELECTRIC FIELD THEREOF EXTENDS IN AN OPPOSITE DIRECTION ALONG THE AXIS OF EACH DIODE. 